Stent with optimal strength and radiopacity characteristics

ABSTRACT

A stent in the form of a thin-walled, multi-cellular tubular structure is provided. The tubular structure has a longitudinal axis and the stent includes a plurality of circumferential sets of strut members. Each set of the strut members is longitudinally displaced from each other and connected to each other by longitudinally extending links. Each set of the strut members forms a closed and cylindrical portion of the stent. Further, each set of the strut members includes a plurality of connected curve sections and diagonal sections. The sets of the strut members further include end sets of strut members located at each end of the stent and central sets of strut members located between the end sets of the strut members. The diagonal sections of the end sets of the strut members have a center portion and two ends. At least one of the diagonal sections of the end sets of the strut members includes a tapered shape with width of one diagonal section is greater at the center of the diagonal section than the width at either end of the diagonal section.

BACKGROUND OF THE INVENTION

[0001] Stents are well known medical devices that are used formaintaining the patency of a large variety of vessels of the human body.A more frequent use is for implantation into the coronary vasculature.Although stents have been used for this purpose for more than ten years,and some current stent designs such as the CORDIS BX Velocity® stent,Cordis Corporation, Miami Lakes, Fla., have the required flexibility andradial rigidity to provide an excellent clinical result, they are notalways clearly seen under standard fluoroscopy.

[0002] Many current tubular stents use a multiplicity of circumferentialsets of strut members connected by either straight longitudinalconnecting links or undulating longitudinal connecting links. Thecircumferential sets of strut members are typically formed from a seriesof diagonal sections connected to curved sections forming a closed-ring,zig-zag structure. This structure opens up as the stent expands to formthe element in the stent that provides structural support for thearterial wall. A single strut member can be thought of as a diagonalsection connected to a curved section within one of the circumferentialsets of strut members. In current stent designs such as the BX Velocity®stent, these sets of strut members are formed from a single piece ofmetal having a uniform wall thickness and generally uniform strut width.Although a stent with uniform width of the strut members will function,if the width is increased to add strength or radiopacity, the sets ofstrut members will experience increased strain upon expansion. Highstrain can cause cracking of the metal and potential fatigue failure ofthe stent under the cyclic stress of a beating heart.

[0003] Existing highly radiopaque stents, such as the gold platedNIROYAL stent sold by Boston Scientific, Inc., Natick Mass., can obscurethe inside of the vessel due to the high radiopacity over the entirelength of the stent. The BeStent sold by Medtronic, Inc., MinneapolisMinn., has small gold markers at the ends of the stent. Those markersonly mark an end point without allowing visualization of the entire endset of strut members.

[0004] Fischell et al, in U.S. Pat. No. 6,086,604, discloses a stentwith the end sets of strut members being gold plated. Such a stent wouldhave ideal radiopacity but may be subject to the corrosive effectsincurred through placement of dissimilar metals in an electrolyticsolution such as blood. There has also been significant evidence thatgold is a poor surface material for stents because it may increase therisk of subacute thrombosis or restenosis. Further, Fischell et al, inU.S. Pat. No. 5,697,971 discloses in its FIG. 7, a stainless steel stentwith increased width diagonal sections in all the circumferential setsof strut members.

SUMMARY OF THE INVENTION

[0005] An ideally radiopaque stent would have end sets of strut membersthat are highly radiopaque so that they can be readily seen, even usinglow power fluoroscopy, and would further contain a central section thatis visible but not too bright so as to obscure the lumen when high powercine film angiograms are taken. The stent should also have only onematerial on its outside surface to avoid potential corrosion; thatmaterial should not promote subacute thrombosis or restenosis.

[0006] The present invention is a stent that is designed to have optimalstrength and radiopacity with good biocompatibility. Unfortunately, thechoices of appropriate biocompatible metals available as thin walltubing for stent construction are somewhat limited. To achieve optimalradiopacity, the stent design of the present invention is adjusted tothe specific radiopacity and strength characteristics of the metal fromwhich the stent is fabricated. What is more, coatings such as parylenemay be needed to avoid corrosion from stents with less biocompatiblematerials and/or dissimilar metals on the stent's outer surface. Ofextreme importance to the present invention is the achievement ofoptimal radiopacity in a stent that ideally is only 0.004 inches wallthickness or less. Such a stent would have a pre-deployment outerdiameter (profile) that would be at least 0.003 inches less thancurrently marketed stents. Ideally, the stent described herein wouldhave a wall thickness between 0.0025 inches and 0.004 inches.

[0007] Described herein are the novel design elements for stents formedfrom the following materials:

[0008] 1. A highly radiopaque metal such as tantalum;

[0009] 2. Metals somewhat more radiopaque than stainless steel, such asthe cobalt based alloy L605;

[0010] 3. Stents coated or plated with highly radiopaque materials likegold; and

[0011] 4. Layered materials such as alternative layers of tantalum andstainless steel.

[0012] 5. The novel design elements that are described herein include:

[0013] 1. Tapered strut width for stents formed from highly radiopaquemetals.

[0014] Although reducing the width of the longitudinally diagonalsection alone will reduce radiopacity without significantly affectingradial strength, by having a taper on the curved sections of thecircumferential sets of strut members, a greatly reduced level of strainupon stent expansion can be achieved without sacrificing radialstrength.

[0015] This is extremely important, as it allows a stent to be made muchstronger than a stent with uniform width of the strut members whilestaying within the same strain limit for the material.

[0016] Tantalum is a metal that has been used in stents; which metal ishighly radiopaque. The optimal radiopacity for a stent design usingtantalum could have uniform width for the circumferential sets of strutmembers and a wall thickness of about 0.0025 inches. To provide moreradial strength and to reduce the probability of the stent ends flaringout during deployment, a wall thickness of about 0.003 inches to 0.035inches would be highly desirable. With uniform width sets of strutmembers, a 0.035 inches wall thickness tantalum stent would be toobright under cine angiography. To reduce the radiopacity of the designwithout significantly impacting the radial strength of the deployedstent, the present invention envisions curved sections and diagonalsections, either or both of which could have a variable or taperedwidth. The curved sections should be tapered (wider at the centercompared to the ends) to reduce strain as previously described. Thelongitudinally diagonal sections can be thinner in the center than atthe ends, to reduce radiopacity for the central sets of strut members.

[0017] It is envisioned that the novel stent described herein might havewider diagonal sections for the end sets of strut members as compared tothe central sets of strut members. This feature would enhance theradiopacity of the end sets of strut members while retaining a moderatelevel of radiopacity for the central sets of strut members. It is alsoenvisioned to have both reduced width diagonals and/or reduced wallthickness for the central sets of strut members. It should be rememberedthat it is fluoroscopic visualization of the end sets of strut membersthat is most important for visualizing stents placed inside a coronaryartery.

[0018] 2. Thicker diagonal sections for metals with radiopacity slightlybetter than stainless steel. The cobalt/tungsten alloy L605 is astronger and more radiopaque metal compared to stainless steel. Toachieve optimal radiopacity using L605 with uniform width sets of strutmembers, the wall thickness is optimally equal to or greater than 0.0045inches. To provide optimal radiopacity with such a metal in stents ofwall thickness 0.004 inches or less, the present invention envisionswider diagonal sections in the sets of strut members. Thus, the tapereddiagonal sections would be wider than the curved sections. The taperedcurved section design for reduced strain may also be highly desirablefor stents made from the L605 alloy.

[0019] 3. End sets of strut members with thinner curved sections. Stentdeliverability into curved coronary arteries is improved when thediagonal sections of the end sets of strut members have a decreasedlength as compared to the length of the diagonal sections of the centralsets of strut members. A shorter length of the diagonal sections willalso reduce outward flaring upon expansion of the stent.

[0020] Decreasing end flaring of the deployed stent is of particularimportance for stents having very thin walls.

[0021] Previous designs that describe a stent with shorter diagonalsections in the end sets of strut members are limited by the strainlimit allowed for the end sets of strut members. As a result, if the endsets of strut members are made as strong as possible while being limitedby the maximum allowable strain for that metal, the central sets ofstrut members will not have optimized radial strength. The presentinvention envisions optimizing the radial strength for all sets of strutmembers, i.e., the metal in all sets of strut members just reach themaximum allowable strain at the limiting diameter for the stent'sexpansion. To achieve this desired attribute, the stent described hereinhas the curved sections of the end sets of strut members being less widethan the curved sections of the central sets of strut members.

[0022] 4. Good side branch arterial access while maintaining small cellsize.

[0023] The stents described herein are typically closed cell stents,having a curved section of a central set of strut members connected toan adjacent set of strut members by a longitudinally extending link. Inone embodiment of the present invention, the circumferential sets ofstrut members are joined by undulating longitudinal connecting linkswith each link having a multiplicity of curved segments so as toincrease the perimeter of the stent's closed cells. One aspect of thepresent invention is that the perimeter of each of the stent's closedcells should be at least 9 mm long. This design parameter allows eachcell of the stent to be expanded to a circular diameter of approximately3 mm (i.e., 9/π mm˜3 mm). This feature allows the “unjailing” of sidebranches of the artery into which the stent is placed. The ideal designto be radially strong, prevent plaque prolapse and still allowsidebranch access will have a maximum deployed cell area of less than0.005 in.² while having a cell perimeter that is at least 9 mm inlength, so as to allow unjailing of side branches. A good cell for sidebranch access should have a perimeter length between 9 mm and 11 mm.(i.e. an expandable circular diameter between 2.86 mm and 3.5 mm). Cellperimeters between 9.5 and 10 mm are optimal.

[0024] 5. Flexible undulating longitudinal links with good supportbetween adjacent sets of strut members. To provide a strong bridgeconnection between adjacent circumferential sets of strut members, theflexible undulating longitudinal connecting links should have nearlyequal extension in the circumferential direction on each side of a linedrawn between the attachment points of the flexible undulatinglongitudinal connecting link to the curved sections of adjacent sets ofstrut members. “N” and inverted “N” shapes for the connecting linksinherently have equal circumferential displacement on each side of theline connecting their attachment points. The specially designed “M” or“W” shapes of the present invention also provide this desirableattribute. Nearly equal circumferential lengths on either side of a linedrawn between the attachment points of the flexible undulatinglongitudinal connecting links help in preventing plaque from pushing the“M” or “W” shaped link inward into the lumen of the stent when the stentis deployed into an artery.

[0025] The “M” and “W” shapes are of particular advantage in obtainingthe desired attribute of small area cells that have good side branchaccess capability because of an increased perimeter length. It shouldalso be understood that the “M” and “W” shapes each add an additionalhalf cycle of undulating link length to the cell perimeter as comparedto an “N” shaped link design, thus improving the stent's longitudinalflexibility. It should also be noted that a “W” link is simply aninverted “M” link.

[0026] 6. Variable thickness radiopaque coatings. The NIROYAL™ stent hasa uniform thickness of gold plating, which makes the center tooradiopaque as compared to the radiopacity of the end sets of strutmembers. Fischell et al., U.S. Pat. No. 6,086,604, teaches stents havinggold placed at the end sets of strut members. This creates a potentialfor corrosion from dissimilar metals, namely, gold and stainless steel.The present invention envisions a gold coating that is sufficientlythick on the end sets of strut members to provide optimal radiopacitywith a thin coating of gold on the rest of the stent. This designprevents obscuring of the arterial lumen while providing an exteriorsurface for the stent that is a single metal, thus avoiding electrolyticcorrosion.

[0027] 7. Polymer coatings for stents coated with gold or havingdissimilar metal surfaces. For stents with non-biocompatible ordissimilar metals, the present invention envisions the use of a polymersuch as parylene to coat the entire outer surface of the stent. Thiswould improve biocompatibility and also allow attachment of organiccompounds such as heparin or phosphorylcholine for reducedthrombogenicity or drugs, such as taxol or rapamycin, for reduced cellproliferation and a decreased rate of restenosis. It is also known thathighly radiopaque materials like tungsten can be mixed into polymers. Astent coating including a plastic with mixed in radiopaque metal couldbe used to enhance both radiopacity and biocompatibility. Such a polymercoating could also be advantageous with a gold coated stent.

[0028] 8. Providing a variable wall thickness. The present inventionalso envisions next generation manufacturing techniques usingphoto-etching, whereby a stent pattern is etched into a thin-walledmetal tube. These techniques already can produce variations in wallthickness as well as strut width for any stent pattern. The presentinvention envisions use of these techniques to create stents withoptimal radiopacity. In particular for a stent formed from a singlemetal or alloy, thicker metal at each end of the stent could increaseradiopacity there as compared to the central section of the stent.Perhaps more important is the use of multi-thickness etching techniqueswith a two- or three- layered tube where one of the layers is a highlyradiopaque material such as tantalum. For example, a two-layer tubehaving one layer of stainless steel and a second layer of tantalum couldbe etched to provide the end sets of strut members with 0.001 inches oftantalum and 0.0025 inches of stainless steel while the remainder of thestent would have less than 0.0005 inches of tantalum with a stainlesssteel layer of 0.003 inches. It is also envisioned that there could betantalum only on the end sets of strut members. Thus, one could producea stent with enhanced radiopacity at the ends with the stent having auniform wall thickness.

[0029] One could even have a stent with increased wall thickness of ametal at the central region of the stent but still having a decreasedradiopacity at that central region if, for example, the stent hadtantalum end struts with stainless steel center struts. Such a stentwould be strongest in the center where the thickest plaque must berestrained.

[0030] It is also envisioned that any of the above optimal radiopacitystent designs may be used with plastic coatings such as parylene,antithrombogenic coatings such as heparin or phosphorylcholine, oranti-proliferative coatings such as taxol or rapamycin.

[0031] Thus it is an object of the present invention to have a stentwith tapered curved sections, the center of the curved sections beingwider than ends of the curved sections so as to reduce plastic strain asthe stent is expanded as compared to a curved section with uniformwidth.

[0032] Another object of the present invention is to have a stent withtapered diagonal sections in the sets of strut members where the centerof the diagonal section is narrower than the ends to reduce theradiopacity of central sets of strut members of the stent as compared toa stent with diagonal sections having a uniform width.

[0033] Still another object of the invention is to have a stent withdecreased wall thickness at the central struts compared to the endstruts so as to have a comparatively higher radiopacity for the end setsof strut members.

[0034] Still another object of the present invention is to have a stentwith tapered diagonal sections for one or more of the sets of strutmembers where the center of the diagonal section is wider than the endsto increase the radiopacity of the end sets of strut members as comparedto a stent with uniform width of the diagonal sections.

[0035] Still another object of the present invention is to have end setsof strut members having both shorter diagonal sections and thinner widthcurved sections as compared to those sections in the central sets ofstrut members.

[0036] Still another object of the present invention is to have atantalum stent with wall thickness less than 0.035 inches having taperedsets of strut members whereby the diagonal sections are less wide thanthe width at the center of the curved sections.

[0037] Still another object of the present invention is to have a closedcell stent design with maximum post-deployment cell area less than 0.005square inches and a cell perimeter length that is equal to or greaterthan 9 mm.

[0038] Still another object of the present invention is to have a stentwith a radiopaque metal coating where the radiopaque metal coating hasgreater wall thickness on the end sets of strut members as compared tothickness on the sets of strut members at the center of the stent.

[0039] Still another object of the present invention is to have a stentetched from a multi-layer metal tube having one layer significantly moreradiopaque than at least one other layer; the etched stent being formedwith increased wall thickness of the more radiopaque layer on the endsets of strut members as compared with the sets of strut members at thecenter of the stent.

[0040] Still another object of the present invention is to have a closedcell stent design with “M” or “W” shaped flexible undulatinglongitudinal connecting links wherein the circumferential extent of theflexible undulating longitudinal connecting links is approximately equalon each side of a line drawn between the proximal and distal attachmentpoints of the flexible undulating longitudinal connecting link.

[0041] Still another object of the present invention is to have thestent with optimized radiopacity formed with an outer surface that isplastic coated to improve biocompatibility.

[0042] Still another object of the present invention is to have thestent with optimized radiopacity that is coated with a plastic materialand an additional organic compound to prevent thrombus formation and/orrestenosis.

[0043] Still another object of the present invention is to have a stentcoated with a plastic material that includes a radiopaque fillermaterial.

[0044] These and other objects and advantages of this invention willbecome apparent to the person of ordinary skill in this art field uponreading of the detailed description of this invention including theassociated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045]FIG. 1 is a flat layout of a prior art stent having uniform strutwidth for the circumferential sets of strut members.

[0046]FIG. 2 is a flat layout of a prior art stent design having “M” and“W” flexible connecting links.

[0047]FIG. 3 is an enlargement of the “M” link of the stent design ofFIG. 2.

[0048]FIG. 4 is an enlargement of the improved “M” link design of thepresent invention.

[0049]FIG. 5 is a flat layout of the present invention stent design fora highly radiopaque metal.

[0050]FIG. 6 is a flat layout of part of the present invention stentdesign of FIG. 5.

[0051]FIG. 7 is a flat layout of an alternate embodiment of part of thepresent invention stent design of FIG. 5.

[0052]FIG. 8 is a flat layout of the present invention stent design fora somewhat radiopaque metal.

[0053]FIG. 9 is a flat layout of the present invention stent design fora stent coated with a radiopaque metal.

[0054]FIG. 10 is a flat layout of an alternate embodiment of the presentinvention stent including an “N” shaped flexible connecting link.

[0055]FIG. 11 is a flat layout of the present invention stent design asphoto-etched from a tube.

[0056]FIG. 12A is an enlargement of a section of the photo-etched stentof FIG. 11.

[0057]FIG. 12B is a longitudinal cross section at 12-12 of the enlargedsection of FIG. 11 shown in FIG. 12A, the stent having a radiopaquecoating that is thickest on the end sets of strut members.

[0058]FIG. 12C is a longitudinal cross section at 12-12 of the enlargedsection of FIG. 11 shown in FIG. 12A, as etched from a two-layer tubewhere one of the tube layers is a moderately radiopaque metal and theother layer is a highly radiopaque metal.

DETAILED DESCRIPTION OF THE DRAWINGS

[0059]FIG. 1 shows a flat layout of an embodiment of a prior art stentdescribed by Fischell et al in U.S. Pat. No. 6,190,403. The stent 5 ofFIG. 1 is shown in its crimped, pre-deployed state as it would appear ifit were cut longitudinally and then laid out into a flat, 2-dimensionalconfiguration. The stent 5 comprises end sets of strut members 2 locatedat each end of the stent 5 and three central sets of strut members 6connected each to the other by sets of longitudinally extendingundulating “N” links 4. The end sets of strut members 2 consist ofalternating curved sections 7 and diagonal sections 9. The central setsof strut members 6 located longitudinally between the end sets of strutmembers 2 consist of alternating curved sections 3 and diagonal sections8. In the prior art stent 5, the longitudinally diagonal sections 9 ofthe end sets of strut members 2 are shorter in length than thelongitudinally diagonal sections 8 of the central sets of strut members6. The shorter diagonal sections 9 will reduce the stiff longitudinallength of metal at the ends of the stent 5 to improve deliverability (byreducing “fish-scaling”) and will also increase the post-expansionstrength of the end sets of strut members 2 as compared with the centralsets of strut members 6. In this prior art stent, the width of thecurved sections 3 and 7 and the diagonal sections 8 and 9 are all thesame. There is no variation in width within any set of strut members orbetween the end sets of strut members 2 and the central sets of strutmembers 6. The stent 5 is a design well suited to stainless steel havinga wall thickness of 0.0045″ or greater, such as found in the CORDIS BXVelocity® stent.

[0060] If the stent 5 were formed from a highly radiopaque metal such astantalum with wall thickness of 0.0030 to 0.0035 inches and with sets ofstrut members 6 having widths of greater than the 0.005 inches that isnecessary for good radial strength, then the stent would be tooradiopaque. In addition, with a wall thickness of 0.003 inches or less,the end sets of strut members 2 might have a tendency to flare outwardlyinto the vessel wall upon expansion. If the end sets of strut members 2are designed to be as strong as possible while not exceeding metalstrain limits at the largest usable diameter of the stent 5, then thecentral sets of strut members 6 with longer diagonal sections 8 will nothave maximized radial strength assuming the same strut width for bothcentral sets of strut members 6 and end sets of strut members 2.Optimized strength at the longitudinal center of a stent is important asit is that region that must typically hold back a larger amount ofplaque than at the ends of the stent.

[0061] One embodiment of the present invention provides that each set ofstrut members should have maximized radial strength rather than havingthe central sets of strut members 6 being less strong than the end setsof strut members as previously described. This design would be similarto the stent 5 of FIG. 1 with the novel improvement being that the widthof the curved sections 3 of the central sets of strut members 6 would begreater than the width of the curved sections 7 of the end sets of strutmembers 2. The greater width of the curved sections 3 will increase thestrength of the central sets of strut members 6 compensating for loss ofradial strength because of the longer diagonal sections 8.

[0062] The stent 60 shown in FIG. 2 is a flat layout of a prior artstent design having “N”, “M” and “W” flexible connecting links. Thestent 60 is shown in its crimped pre-deployed state as it would appearif it were cut longitudinally and then laid out into a flat,2-dimensional configuration. It should be clearly understood that thestent 60 is in fact cylindrical in shape, which cylindrical shape wouldbe obtained by rolling the flat configuration of FIG. 2 into a cylinderwith the top points “G” joined to the bottom points “H”. The stent 60 istypically fabricated by laser machining of a cylindrical, stainlesssteel tube.

[0063] A central set of strut members 62 is a cylindrical, closed,ring-like section of the stent 60 consisting of a multiplicity of curvedsections 63 connected to diagonal sections 68. Every curved section 63of each central set of strut members 62 is attached to a connecting linkwhich is either a flexible “N” link 44, “M” link 64 or a “W” link 84.The stent 60 also has two end sets of strut members 72 consisting of amultiplicity of curved sections 73 connected to diagonal sections 78. Inthis embodiment, half of the curved sections 73 of the end set of strutmembers 72 are attached to “N” links 44 with the other half of thecurved sections 73 situated at the extreme ends of the stent 60. Thediagonal sections 78 of the end sets of strut members 72 are shorterthan the diagonal sections 68 of the central sets of strut members 62.Shorter diagonal sections enhance the post-expansion radial strength ofthe end sets of strut members 72 as compared to the central sets ofstrut members 62.

[0064]FIG. 3 is an enlargement of the “M” link 64 of the prior art stentof FIG. 2. One disadvantage of this design relates to thecircumferential extent of the “M” link 64 with respect to a line 65 thatcould be drawn between the two attachment points 68 where the “M” link64 attaches to the curved sections 63. Because almost all of the “M”link 64 lies above the line 65, pressure on the top of the “M” link 64from plaque in an artery could bend the top of the “M” link 64 inwardinto the arterial lumen. This would be highly undesirable. Ideally, an“M” or “W” link should have an equal circumferential extent on eitherside of a line drawn between the attachment points to adjacent sets ofstrut members as shown in FIG. 4.

[0065] One aspect of the present invention is an improved “M” link 14 asshown in FIG. 4. The “M” link 14 has a circumferential extent (i.e.,length) L′ above and L″ below the line 15. The line 15 is drawn betweenthe attachment points 18 where the “M” link 14 attaches to adjacentcurved sections 13. Such a balanced design would diminish any likelihoodof the flexible connecting link 14 from expanding into the arteriallumen.

[0066]FIG. 5 is a flat layout view of a stent 20 that includes someembodiments of the present invention. The design of FIG. 5 isparticularly applicable to stents made from a highly radiopaque metalsuch as tantalum. The stent 20 of FIG. 5 is shown in flat, layout viewbased on its pre-deployed state, as it would appear before it is crimpedonto a balloon catheter. The stent 20 comprises end sets of strutmembers 22 located at each end of the stent 20 and central sets of strutmembers 26 connected each to the other by sets of individual flexible“M” links 24. The “M” links 24 are similar to the “M” link 14 of FIG. 4.The end sets of strut members 22 consist of a multiplicity of curvedsections 27 connected to diagonal sections 29. The central sets of strutmembers 26 located longitudinally between the end sets of strut members22 consist of a multiplicity of curved sections 23 connected to diagonalsections 28.

[0067] One can also define a strut element 25 as being composed of oneadjacent curved section 23 joined to a diagonal section 28. As seen inFIG. 5, it is clear that one can describe a central set of strut members26 as being a closed, circumferential, ring-like structure comprising amultiplicity of connected strut elements 25. An end set of strut memberscould be likewise defined as being a multiplicity of connected strutelements 17.

[0068] The stent 20 is a closed cell stent having cells 19 formed fromportions of adjacent sets of strut members connected by “M” links 24.For coronary arteries, prolapse of plaque into the arterial lumen willbe minimized if the area within the cell 19 does not exceed 0.005 squareinches at all diameters up to the maximum deployment diameter of thestent 20. An important aspect of stent design is to be able to place aguidewire through the expanded cell 19, into a side branch vessel. Aballoon angioplasty catheter can then be advanced over the guidewire andinflated to enlarge and circularize the opening of the cell 19 to“unjail” the side branch vessel. By “unjailing” is meant removing metalfrom the ostium of the side branch vessel, thus improving blood flow tothat side branch. One concept of the present invention is that the cell19 has an interior length of the perimeter that is at least 9 mm. Sinceballoon dilatation of the cell 19 would cause it to be near circular, aninside perimeter length around inside of the cell 19 would provide aninside diameter of 9/π, which is approximately 3 mm. A good cell designfor side branch access should have an inside perimeter length between 9mm and 11 mm. (i.e., an expanded inside circular diameter between 2.86and 3.5 mm) where cell perimeters between 9.5 and 10 mm are optimal andwould be suitable for essentially any side branch of a coronary artery.

[0069] In the stent 20, the diagonal sections 29 of the end sets ofstrut members 22 are shorter in length than the diagonal sections 28 ofthe central sets of strut members 26. The shorter diagonal sections 29will reduce the longitudinal extent of the metal strut at the end of thestent to improve deliverability into a vessel of the human body bydecreasing fish-scaling. In the stent 20, the width of the curvedsections 23 and 27 and the diagonal sections 28 and 29 are different ascompared to the prior art stents 5 and 6 of FIGS. 1 and 2.

[0070] The exact design of the stent 20 is most clearly seen in theexpanded view of the stent section 21 of FIG. 5 as shown enlarged inFIG. 6. FIG. 6 shows that the curved sections 23 (of the central sets ofstrut members 26 of FIG. 5) have a width at the center of the curveW_(c). The width of the curved sections 23 taper down as one moves awayfrom the center of the curve until a minimum width W_(d) is reached atthe center of the section 28. To achieve this taper, the inside arc ofthe curved section 23 has a center that is longitudinally displaced fromthe center of the outside arc. This tapered shape for the curved section23 provides a significant reduction in metal strain with little effecton the radial strength of the expanded stent as compared to a stenthaving sets of strut members with a uniform strut width.

[0071] This reduced strain design has several advantages. First, it canallow the present invention design to have a much greater usable rangeof radial expansion as compared to a stent with a uniform strut width.Second, it can allow the width at the center of the curve to beincreased which increases radial strength without greatly increasing themetal strain (i.e. one can make a stronger stent). Finally, the taperreduces the amount of metal in the stent and that should improve thestent thrombogenicity.

[0072]FIG. 6 also shows a unique design for the end sets of strutmembers 22. The diagonal sections of the end sets of strut members 22have a length L_(end) that is shorter than the length L of the diagonalsections 28 of the central sets of strut members 26. To maximize theradial strength of a stent along its entire length, each set of strutmembers should just reach the maximum allowable plastic strain for themetal being used at the largest allowable expanded diameter of thestent. In the stent of FIG. 1, the curved sections 7 of the end sets ofstrut members 2 and the curved sections 3 of the central sets of strutmembers 6 have the same widths. As a result, the end sets of strutmembers 2 (which have shorter diagonal sections 9) will reach themaximum allowable diameter at a level of strain that is greater than thelevel of strain experienced by the central sets of strut members 6.

[0073] An optimum strength stent design would have the same strain atthe maximum stent diameter for both the end sets of strut members 2 andthe central sets of strut members 6. For the stent design of FIGS. 5 and6, one desires to have the end sets of strut members 22 reach themaximum strain limit at the same stent diameter as the central sets ofstrut members 26. The present invention teaches a design with the widthat the center of the curve WC_(d) _(—) _(end) of the curved section 27being less than the width W_(c) of the curved sections 23 of the centralsets of strut members 26. This reduced width for the curved sections 23compensates for the shorter length L_(end) of the end diagonal sections29 so that there is the same strain in both the central and end sets ofstrut members 22 and 26 respectively as the stent 20 is expanded to itsmaximum allowable diameter.

[0074] The end sets of strut members 22 can also be tapered like thecentral sets of strut members 26 where the width of the strut tapersdown as one moves away from the center of the curve of the curvedsections 27 until a minimum width W_(d) _(—) _(end) is reached at thediagonal section 29. The curved sections 23, 27 each have an inside(concave) arc and an outside (convex) arc. Each arc has a center that islongitudinally displaced from the other center.

[0075] The tapered strut design shown in FIGS. 5 and 6 also has anadvantage for stents made from highly radiopaque metals such astantalum. If one uses uniform strut width as seen with the stent 5 ofFIG. 1, then a properly designed thin-walled (0.0025 inches to 0.035inches) wall tantalum stent may be too radiopaque. The reduced metalfrom the thinner diagonal sections 28 and 29 will decrease theradiopacity without affecting radial strength. Nominal dimensions anddimension ranges (all in inches) for a tantalum stent produced using thedesign of FIG. 5 are as follows: Element Nominal Range W_(c) 0.0060.0045 to 0.007  W_(d) 0.0045 0.035 to 0.005 W_(c) _(—) _(end) 0.00450.004 to 0.005 W_(d) _(—) _(end) 0.0045 0.035 to 0.005 L 0.028 0.020 to0.030 L_(end) 0.025 0.015 to 0.026 Wall Thickness 0.003 0.0025 to 0.035 

[0076] Although the present invention shows the “M” shaped flexible link24 being used, the present invention strut designs will function withany link shape including “N”, “W”, “S”“U”, “V” and inverted “N”, “U” and“V” designs. It should also be noted that the “M” link 24 shown in FIG.6 has exactly five longitudinally extending curved segments 24A, 24B,24C, 24D and 24E.

[0077]FIG. 7 is an alternative embodiment 21′ of section 21 shown inFIG. 6 of the present invention stent 20 of FIG. 5. In this embodiment,the only difference is the shape of the diagonal sections 28′. Thediagonal sections 28 of FIG. 6 have uniform thickness. The diagonalsections 28′ of FIG. 7 are tapered from a width Wd_(d)″ at the end ofthe diagonal section 28′ where it connects to the curved sections 23′ toa width W_(d)′ at the center of the diagonal section 28′. The advantageof the inward taper of the diagonal sections 28′ is that removal of moremetal will reduce the radiopacity of the longitudinal center region ofthe stent 20 as compared to a stent with uniform width diagonal sections28 as seen in FIG. 6. The additional taper may also further reduce themetal strain as the stent is expanded. Although one could taper thediagonal sections 29 of the end sets of strut members 22 of FIG. 5,there is an advantage in having the end sets of strut members 22 beingmore radiopaque than the central sets of strut members 26. This isbecause visualization of the stent ends is the most important aspect ofradiopacity for a stent. Therefore, a preferred embodiment of thepresent invention is as seen in FIG. 7 to have tapered diagonal sections28′ in the central sets of strut members 26 and uniform thicknessdiagonal sections 29 (having a greater average width) for the end setsof strut members 22.

[0078] Instead of connecting every curved section with a flexible link,an alternate embodiment may use straight links connecting only half ofthe curved sections of the sets of strut members. Such a stent couldalso have the advantage of a reduced strain strut design as shown inFIGS. 5, 6 and 7.

[0079] For the stent of FIG. 5, it should also be understood that thewall thickness of the central set of strut members 26 could be thinnerthat the wall thickness of the end set of strut members 22. Also itshould be noted that the “M” links 24 also have a much narrower width ascompared to the width of any strut member of the end set of strutmembers. Both these attributes of the stent 20 create the followingdesirable radiopacity characteristics: highly radiopaque end sets ofstrut members and decreased radiopacity at the central region of thestent 20.

[0080]FIG. 8 is a flat layout view of another embodiment of the presentinvention showing a stent 30 made from a moderately radiopaque metalsuch as the cobalt-tungsten alloy L605. The alloy L605 has great radialstrength and is approximately 20% to 30% more radiopaque than stainlesssteel. Therefore, with L605, the same level of radiopacity is achievedwith a stent wall thickness that is 20% to 30% less than a stent madefrom stainless steel. One goal in the use of L605 would be to reduce thewall thickness by 30% but end up with a stent that is still moreradiopaque than an equivalent stainless steel stent such as the stent 5shown in FIG. 1.

[0081] The stent 30 of FIG. 8 is shown in a layout view based on itspre-deployed state, as it would appear before it is crimped onto aballoon catheter. The stent 30 comprises end sets of strut members 32located at each end of the stent 30 and central sets of strut members 36connected each to the other by sets of flexible “M” links 34. The “M”links 34 are similar to the “M” links 14 of FIG. 4. Each end set ofstrut members 32 comprises alternating curved sections 37 and diagonalsections 39 connected together to form a closed circumferentialstructure. The central sets of strut members 36 located longitudinallybetween the end sets of strut members 32 comprises curved sections 33and diagonal sections 38 connected together to form a closedcircumferential ring-like structure.

[0082] In the stent 30, the diagonal sections 39 of the end sets ofstrut members 32 are shorter in length than the diagonal sections 38 ofthe central sets of strut members 36. The shorter diagonal sections 39will reduce the longitudinal length of metal at the end of the stent toimprove deliverability into a vessel of the human body. In the stent 30,the widths of the diagonal sections 38 and 39 are different as comparedto the prior art stents 5 and 60 of FIGS. 1 and 2.

[0083] The novel concepts of the stent of FIG. 8 are shown most clearlyin the expanded view of the stent section 31 shown in FIG. 9. In FIG. 9it can be seen that the diagonal sections 38 of the central sets ofstrut members 36 have a width at the center T_(c), and a width at theend T_(e) where the width in the center T_(c) is larger than the widthat the end T_(e). This allows for increased radiopacity withoutaffecting the design of curved sections 33 that are the primary stentelements involved for stent expansion. The curved sections 33 and 37shown in FIG. 9 are tapered similar to the curved sections 23 and 27 ofFIG. 6. It is also envisioned that the curved sections 33 and 37 couldhave uniform width similar to the curved sections 3 and 7 of FIG. 1. Thediagonal sections 39 of the end sets of strut members 32 also have atapered shape. The diagonal sections 37 have a width in the center T_(c)_(—) _(end) and a width at the end T_(e) _(—) _(end) where the width inthe center T_(c) _(—) _(end) is larger than the width at the end T_(e)_(—) _(end). Because of the desire for the end sets of strut members 32to be the most radiopaque part of the stent 30, the diagonal section 39center width T_(c) _(—) _(end) of the end sets of strut members 32 isshown in FIG. 9 to be wider than the width T_(c) of the diagonal section38. A wider piece of metal will be more radiopaque. Thus, the stent hascurved sections with a single bend connecting the diagonal sections ofits sets of strut members, and flexible connecting links connecting thecurved sections of its circumferential sets of strut members.

[0084] The stent of FIG. 10 is an alternate embodiment of the presentinvention showing central sets of strut members 46 having curvedsections 43 and diagonal sections 48 with tapered shapes similar indesign to the curved sections 23′ and diagonal sections 28′ of the stentsection 21′ shown in FIG. 7. The stent 40 of FIG. 10 is shown in alayout view in its pre-deployed state as it would appear before it iscrimped onto a balloon catheter. The stent 40 comprises end sets ofstrut members 42 located at each end of the stent 40 and central sets ofstrut members 46. The sets of strut members 42 and 46 are connected eachto the other by sets of individual flexible “N” links 44. The “N” links44 are similar in shape but slightly longer than the “N” links 4 ofFIG. 1. The end sets of strut members 42 consist of curved sections 47and diagonal sections 49. The central sets of strut members 46 locatedlongitudinally between the end sets of strut members 42 consist ofcurved sections 43 and diagonal sections 48.

[0085] The stent 40 is a closed cell stent having cells 45 formed fromportions of adjacent sets of strut members connected by “N” links 44.Prolapse of plaque through the closed cells 45 is minimized if theexpanded area of the cell 45 is less than about 0.005 in.² at anydiameter up to the maximum deployment diameter of the stent 40. It isalso important for an optimum stent design that a guidewire can beplaced through the expanded cell 45 into a side branch vessel. A balloonangioplasty catheter would then be advanced over the guidewire, throughthe cell 45 and inflated to “unjail” the side branch, i.e. remove anystent strut that is blocking blood flow into that side branch. Thepresent invention design should have an interior perimeter of the cell45 that is at least 9 mm, thus allowing a nearly 3 mm diameter circularopening to be achieved for unjailing.

[0086]FIG. 11 is a flat layout view of another embodiment of the presentinvention in the form of a stent 50 that is photo-etched from a metaltube. The stent 50 is shown in its pre-deployed state as it would appearbefore it is crimped onto a balloon catheter. The stent 50 comprises endsets of strut members 52P and 52D located respectively at the proximaland distal ends of the stent 50. The stent 50 also has central sets ofstrut members 56 connected each to the other by sets of flexible “M”links 54. The “M” links 54 are similar to the “M” links 14 of FIG. 4.The end sets of strut members 52P and 52D each consists of curvedsections 57 and diagonal sections 59. The central sets of strut members56 located longitudinally between the end sets of strut members 52consist of curved sections 53 and diagonal sections 58.

[0087] The section 55 of the photo-etched stent 50 is shown enlarged inFIG. 12A. The FIGS. 12B and 12C show two embodiments of the presentinvention that can provide a stent with enhanced radiopacity at thestent ends.

[0088]FIG. 12A shows diagonal sections 58 and 59 and an “M” link 54connecting the curved sections 53 and 57.

[0089]FIG. 12B is a longitudinal cross section at 12-12 of the stentsection 55 shown in FIG. 12A. The stent design shown in FIG. 12B has ahighly radiopaque coating that is thicker on the end sets of strutmembers 52 as compared to the thickness on either the flex links 54 orthe central sets of strut members 56. FIG. 12B shows the coating 57C onthe curved section 57 of the end set of strut members 52 being thickerthan the coating 54C on the flex link 54 and also thicker than thecoating 53C on the curved section 53. The most likely coating for thestent 50 would be gold plating although platinum, tantalum or any otherhighly radiopaque metal could be used.

[0090] The present invention has the entire stent coated to provide anexterior surface for the stent 50 that is formed from a single metal.This reduces the potential for corrosion that can occur with dissimilarmetals on the stent's exterior surface when the stent is placed in asaline solution such as blood.

[0091] It is also envisioned that even with the entire stent coated witha highly radiopaque metal, an additional coating of a flexible plasticsuch as parylene may be desirable. Such an organic coating has theadditional advantage of allowing the attachment of drugs such as taxolor rapamycin to reduce restenosis. Techniques for gold plating metalssuch as stainless steel and controlling the thickness of the plating arewell known in the art of metal plating.

[0092]FIG. 12C is the longitudinal cross section at 12-12 of yet anotheralternate embodiment of the enlarged section 55 of FIG. 11 shown in FIG.12A. The stent design shown in FIG. 12C is etched from a two-layer tubewhere one of the tube layers is a metal of conventional radiopacity suchas stainless steel and the other layer is a highly radiopaque metal suchas tantalum. Although the total wall thickness of the stent of thisembodiment remains nearly constant, the end sets of strut members 52′have a thicker layer of the radiopaque metal than the flex links 54′orthe central sets of strut members 56′. The curved section 57′ of the endset of strut members 52′ has conventional metal layer 57N′ andradiopaque metal layer 57R′. The flex link 54′ has a standard metallayer 54N′ and a radiopaque metal layer 54R′. The central sets of strutmembers 56′ have curved sections 53′ with conventional metal layers 53N′and radiopaque metal layers 53R′.

[0093] It can be seen from FIG. 12C that the radiopaque metal layer 57R′of the end sets of strut members 52′ is thicker than the radiopaquemetal layers 54R′ and 53R′. In recent years, multi-layer photo-etchingprocesses for metals that can control the thickness of individual layershave been developed so that the embodiment of FIG. 12C can be producedwithin the current state of the art of photo-etching. Using thisapproach, two and three layer tubing is now available from severalmanufacturers and can be photo-etched to make a stent with an optimaldesign which is high radiopacity for the end set of strut members andreduced radiopacity for the central sets of strut members. Specifically,a stent with the characteristics as seen in FIG. 12B or FIG. 12C wouldhave the desirable attribute of end sets of strut members with greaterradiopacity than the remainder of the stent.

[0094] Various other modifications, adaptations, and alternative designsare of course possible in light of the above teachings. Therefore, itshould be understood at this time that within the scope of the appendedclaims the invention may be practiced otherwise than as specificallydescribed herein.

What is claimed is:
 1. A deployed stent in the form of a thin-walled,multi-cellular, tubular structure having a longitudinal axis, the stentcomprising: a multiplicity of circumferential sets of strut members,each set of strut members being longitudinally separated each from theother and each set of strut members forming a closed, ring-likecylindrical portion of the stent, each set of strut members consistingof a multiplicity of strut elements, each strut element consisting ofone curved section joined at a junction point to one diagonal sectionwith each junction point being an end point of each curved section; amultiplicity of generally longitudinally disposed sets of flexible linkswith each set of flexible links connecting two of the multiplicity ofcircumferential sets of strut members, each set of flexible linksconsisting of a multiplicity of individual flexible links, eachindividual flexible link being a single undulating structure thatextends generally in the longitudinal direction that is parallel to thestent's longitudinal axis and at least one flexible link being selectedfrom the group that includes “M” links and “W” links; and the sets ofstrut members and connecting flexible links together forming amultiplicity of closed perimeter cells, at least half of all closedperimeter cells having an inside perimeter length greater than 9 mm. 2.The deployed stent of claim 1 wherein at least half of the closedperimeter cells having an inside area of less than 0.005 square inchesat the designed limit of expansion for the stent.
 3. The deployed stentof claim 2 wherein the shape of at least one of the individual flexiblelinks is selected from a group that includes “N” shaped links andinverted “N” shaped links, each of said links having at least fourgenerally longitudinal extending curved segments.
 4. The deployed stentof claim 1 wherein at least half of the closed perimeter cells have aninside metal perimeter length that is less than 11 mm.
 5. A stent in theform of a thin-walled, multi-cellular, tubular structure having alongitudinal axis, the stent comprising: a multiplicity ofcircumferential sets of strut members, each set of strut members beinglongitudinally separated each from the other and each set of strutmembers forming a closed, ring-like cylindrical portion of the stent,each set of strut members consisting of a multiplicity of strutelements, each strut element consisting of one curved section joined ata junction point to one diagonal section; and a multiplicity of sets offlexible links with each set of flexible links connecting two of themultiplicity of sets of strut members, each set of flexible linksconsisting of a multiplicity of individual flexible links, eachindividual flexible link being a single undulating structure thatextends generally in the longitudinal direction that is parallel to thestent's longitudinal axis and each individual flexible link having twoends that are fixedly attached to an adjacent set of strut members; theshape of at least some of the flexible links being selected from a groupthat includes “M” links and “W” links, wherein each of said links haveat least five generally longitudinally extending curved segments, eachflexible link having a proximal attachment point to a curved section ofone circumferential set of strut members and a distal attachment pointto a curved section of a second circumferential set of strut members,each individual flexible link having a maximum circumferential extentthat is approximately the same as measured from each side of a linedrawn between the proximal attachment point and the distal attachmentpoint of that individual flexible link.
 6. A stent in the form of athin-walled, multi-cellular, tubular structure having a longitudinalaxis, the stent comprising a multiplicity of circumferential sets ofstrut members, each set of strut members being longitudinally separatedeach from the other and each set of strut members forming a closed,cylindrical portion of the stent, each set of strut members comprising amultiplicity of connected curved sections and diagonal sections, thesets of strut members including end sets of strut members located ateach end of the stent and central sets of strut members positionedbetween the end sets of strut members, the diagonal sections of the endsets of strut members having a generally greater width than the diagonalsections of the central sets of strut members so as to improve theradiopacity of the end sets of strut members as compared to theradiopacity of the central sets of strut members.
 7. A stent in the formof a thin-walled, multi-cellular, tubular structure having alongitudinal axis, the stent comprising a multiplicity ofcircumferential sets of strut members, each set of strut members beinglongitudinally separated each from the other and each set of strutmembers forming a closed, cylindrical portion of the stent, each set ofstrut members comprising a multiplicity of connected curved sections anddiagonal sections, the sets of strut members including end sets of strutmembers located at each end of the stent and central sets of strutmembers positioned between the end sets of strut members, the curvedsections of the central sets of strut members having a generally greaterwidth than the width of the curved sections of the end sets of strutmembers and the diagonal sections of the central sets of strut membershaving a greater length as compared to the length of the diagonalsections of the end sets of strut members so as to provide approximatelymatched radial strength for the central sets of strut members and theend sets of strut members.
 8. The stent of claim 7 wherein the width ofthe curved sections of the central sets of strut members is at least0.0005 inch greater than the width of the curved sections of the endsets of strut members.
 9. The stent of claim 7 wherein the length of thediagonal sections of the central sets of strut members is at least 0.001inch greater than the length of the diagonal sections of the end sets ofstrut members.
 10. A stent in the form of a thin-walled, multi-cellular,tubular structure having a longitudinal axis, the stent comprising amultiplicity of circumferential sets of strut members, each set of strutmembers being longitudinally separated each from the other and connectedeach to the other by one or more longitudinally extending links, eachset of strut members forming a closed, cylindrical portion of the stent,each set of strut members comprising a multiplicity of connected curvedsections and diagonal sections, the sets of strut members including endsets of strut members located at each end of the stent and central setsof strut members positioned between the end sets of strut members, theend sets of strut members having greater wall thickness than the wallthickness of the central sets of strut members so as to increase theradiopacity of the end sets of strut members.
 11. A stent in the form ofa thin-walled, multi-cellular, tubular structure having a longitudinalaxis, the stent comprising a multiplicity of circumferential sets ofstrut members, each set of strut members being longitudinally separatedeach from the other and connected each to the other by one or morelongitudinally extending links, each set of strut members forming aclosed, cylindrical portion of the stent, each set of strut memberscomprising a multiplicity of connected curved sections and diagonalsections, the sets of strut members including end sets of strut memberslocated at each end of the stent and central sets of strut memberspositioned between the end sets of strut members, each of the sets ofstrut members being coated with a highly radiopaque metal, the end setsof strut members having a greater wall thickness of the highlyradiopaque coating as compared to a lesser thickness of the radiopaquecoating on the central sets of strut members so as to have an increasedradiopacity of the end sets of strut members.
 12. The stent of claim 11wherein the highly radiopaque metal is gold.
 13. The stent of claim 11wherein the highly radiopaque metal is coated with a plastic material.14. The stent of claim 11 wherein the plastic coating is parylene.
 15. Astent in the form of a thin-walled, multi-cellular, tubular structurehaving a longitudinal axis, the stent comprising a multiplicity ofcircumferential sets of strut members, each set of strut members beinglongitudinally separated each from the other and connected each to theother by one or more longitudinally extending links, each set of strutmembers forming a closed, cylindrical portion of the stent, each set ofstrut members comprising a multiplicity of connected curved sections anddiagonal sections, the sets of strut members including end sets of strutmembers located at each end of the stent and central sets of strutmembers positioned between the end sets of strut members, the diagonalsections of the central sets of strut members have a center and twoends, at least one of the diagonal sections of the central sets of strutmembers having a tapered shape wherein the width of the at least onediagonal section is different at the center of the diagonal section ascompared to the width at either end of that diagonal section.
 16. Thestent of claim 15 wherein the width of the at least one diagonal sectionis less at the center of that diagonal section compared to the width ateither end of that diagonal section.
 17. The stent of claim 15 whereinthe width of the at least one diagonal section is greater at the centerof that diagonal section as compared to the width at either end of thatdiagonal section.
 18. The stent of claim 15 wherein the diagonalsections of the end sets of strut members have a center and two ends, atleast one of the diagonal sections of the end sets of strut members hasa tapered shape wherein the width of the at least one diagonal sectionis greater at the center of the diagonal section as compared to thewidth at either end of that diagonal section.
 19. A stent in the form ofa thin-walled, multi-cellular, tubular structure having a longitudinalaxis, the stent comprising a multiplicity of circumferential sets ofstrut members, each set of strut members being longitudinally separatedeach from the other and connected each to the other by one or morelongitudinally extending links, each set of strut members forming aclosed, cylindrical portion of the stent, each set of strut memberscomprising a multiplicity of connected curved sections and diagonalsections, the sets of strut members including end sets of strut memberslocated at each end of the stent and central sets of strut memberspositioned between the end sets of strut members, the diagonal sectionsof the end sets of strut members have a center and two ends, at leastone of the diagonal sections of the end sets of strut members has atapered shape wherein the width of the at least one diagonal section isgreater at the center of the diagonal section as compared to the widthat either end of that diagonal section.